To manufacture an object from plaster, one mixes hydratable calcium sulphate (CaSO.sub.4) with water, one shapes the object, the calcium sulphate is hydrated, is converted to double hydrate and simultaneously crystallized. In order to be able to shape the object, a quantity of water, greater than the quantity strictly necessary for the hydration of the CaSO.sub.4 is often used, and after the shaping, this excess water must be eliminated.
It is a widespread practice to remove most of this water, before setting begins, by a mechanical process of the type: free flow, pressing, then waiting a certain amount of time for the plaster to set and for the hydration water to be used up, after which the object is subjected to a "thermic drying" for several hours in a "thermic dryer" in order to eliminate any free remaining water.
Standardly, the air to which the plaster objects are subjected in the dryers during the thermic drying is at a constant temperature, low, preferably less than 70.degree. C. or an initially higher temperature (150.degree. C.) but which is gradually lowered in such a way that the plaster remains below the temperature which would deteriorate the formed double hydrate.
Under these conditions, the drying time is long--several hours, even several days--because, on the one hand, the temperature is low and because, on the other hand, the excess water trapped in the mesh of CaSO.sub.4.2H.sub.2 O crystals is difficult to extract, resulting in heavy, important and costly installations.
Moreover, determining the optimal drying time and correct drying temperature for each stage is a delicate point, because of the risk of deterioration to the plaster. The heat acts on the surface of the product being dried and the temperature must be adjusted in such a way that there is always water at the surface. At the start of the drying there is indeed water at the surface, but afterwards the water at the heart of the product must have the time to move to the surface at the risk of damage to the outer portion of the plaster. The speed of elimination of the water is more important on the edges of the boards than anywhere else, whence the necessity to protect them with a screen for example, or to diminish the heat or the output of hot air reaching them in order to avoid damage to the plaster at the edges.